Head suspension assemblies (HSAs) in rotatable data storage devices are spring structures that perform the difficult task of holding and positioning a head assembly nanometers away from the rapidly spinning irregular surface of a rotatable data storage device. The HSA can be part of a magnetic hard disk drive, the most common type of disk drive today, or of another type of drive such as an optical disk drive.
A HSA comprises different elements, the most common being a suspension assembly and a head assembly. A suspension assembly, the spring element, usually includes a load beam and a gimbal, each composed of a carefully balanced combination of rigid regions and flexible spring regions. A typical head assembly includes a "head", comprising a highly sensitive read/write transducer, that is attached to an air bearing slider.
In a magnetic disk drive, a write transducer transforms electrical pulses to small magnetic fields which it then "writes" on a magnetic disk. A read transducer decodes these magnetic fields back into electrical pulses. The order of the magnetic fields and their subsequent orientation, aligned along the circumference of the disk in north-south configuration, defines a bit code that the transducer detects as the head floats on a cushion of air over the magnetic disk.
A head suspension assembly generally attaches at its proximal end to a rigid arm manipulated by a linear or rotary motion actuator designed to locate the head at any radial position above the disc. The spinning disk coupled with the actuator movement allows the head to gain access to multiple tracks across the disk surface, each track capable of containing large amounts of densely stored data. Positioned at the distal end of a suspension assembly, a gimbal holds the head assembly level and at a constant distance over the contours of the disk.
The closer the head assembly can fly to the surface of a magnetic disk, the more densely can information be stored (the strength of a magnetic field is inversely proportional to the square of the distance, thus the closer the head flies the smaller the magnetic "spot" of information). Today's disk drives strive to reach fly ing clearances close to 100 nanometers=0.1 micrometers (a human hair is about 100 micrometers thick). Greater data densities allow for greater storage and smaller size. But the head assembly must not touch the disk ("crash"), as the impact with the rapidly spinning disk (rotating at about 3600 rpm or faster) could destroy both the head and the surface of the disk, along with the data stored on it.
In order to achieve this delicate and precise positioning, a suspension assembly must carefully balance the load applied to the head assembly against the upward lift of the air stream on the slider. Since at this microscopic level the seemingly smooth surface of the disk is full of peaks and valleys, the HSA must be very responsive in order to maintain a level flying height. To avoid delays and errors, the HSA must be low in mass and both rigid to resist inertial movement and vibration and flexible to adapt to the undulations on the surface of the disk. Given the minute tolerances involved, even a small unexpected change in the loads or biases within the HSA, and specially in the flexibility of the spring regions, may cause a destructive crash.
As the head writes and reads to and from the disk, it receives and sends electrical pulses of encoded information. These electrical signals are amplified and processed by appropriate circuitry. Signal transmission requires conductors between the dynamic "flying" slider and the static circuitry of the data channels. However, while conductors can be routed fairly easily over the rigid actuator arm, providing the final interconnections through the suspension assembly and to the head is often extremely troublesome with current interconnect schemes.
Specially designed HSA interconnect assemblies are required in order to relay electrical signals accurately while not disturbing the precise balance of the HSA components. The term interconnect assembly refers to the entire interconnect system, including conductors, insulators, and other features. In order to assure data precision, interconnect assemblies must transmit the electrical signals free from interference or signal loss due to high inductance, high capacitance, and/or high resistance. Optimal interconnect conductors must be attached securely in order to reduce movement and vibration which cause varying electrical characteristics and affect mechanical performance. They must have low resistance and be well insulated from electrical ground.
Also, as technology advances, an interconnect system also must be capable of handling a plurality of signals. A basic interconnect assembly for a transducer having a single read/write inductive element calls for two conductors. More complex transducer designs may incorporate a separate magneto-resistive read element and an inductive write element, thus requiring a minimum of 3 conductors if the elements are tied together or a minimum of 4 conductors if the elements are completely separate. Other systems require even more conductors.
The problem is that in a HSA, interconnect assemblies must be planned around competing and limiting design considerations. The interaction of all the elements of an HSA forms a carefully balanced system. An ideal interconnect assembly must satisfy strict mechanical and manufacturing requirements.
First, an interconnect assembly must not impose unpredictable loads and biases which might alter the exact positioning of the head assembly, nor must it detract from the ability of the spring regions to adjust to variations in the surface of the disk, vibrations, and movement. Alterations to the flying height of the head can significantly affect data density and accuracy and even destroy the system in a crash. Neither of these two results would be well received by computer users.
Rigidity increases in relation to the third power of cross-sectional thickness. To respond to air stream changes and to hold a floating head, suspension assemblies are very thin and light, specially around the sensitive spring areas. A thick conductor placed atop of a thin suspension will dramatically increase a spring region's stiffness. Moreover, overshoot errors caused by inertial momentum are also affected by thick, high-in-mass conductors. Therefore, the ideal interconnect assembly must be low in mass and be very thin. As an additional requirement, interconnects placed over spring regions must not plastically deform when the region flexes, since that would hinder the return of the spring to its normal position and apply a load on the suspension assembly.
A second design consideration comes from manufacturing constraints. Like any commercial product, interconnect assemblies must be efficient to manufacture. Additionally, they must mate well and easily with th e suspension assembly and the head assembly, be resistant to damage, and have precise manufacturing tolerances. Complex shaping and mounting processes are costly and decrease the reliability of the whole HSA manufacturing process. Fragile electrical conductors or interconnects that have to be added to the suspension assembly early in the manufacturing process are more susceptible to damage by subsequent production steps. They drastically diminish the manufacturing useful output yield.
Exacting tolerances are necessary to avoid defects and unpredictable loads and biases, specially when dealing with such minute measurements and clearances. During the process of manufacturing and assembling the HSA and of attaching an interconnect system, any lack of precision in forming or assembling the individual elements contributes to a lack of planarity in the surfaces of the elements. A buildup of such deviations from tolerance limits in the individual elements can cause deviation from desired planar parallelism in the final head suspension assembly. The parameters of static roll and static pitch torque in the final HSA result from these inherent manufacturing and assembly tolerance buildups. As the industry transitions to smaller slider/transducer sizes to increase data storage density, limitations of the current interconnecting devices increase the potential for read/write errors and impose ceilings on data storage density.
In conclusion, as today's data density requirements necessitate for the head to fly even closer to the disk, there is less room for imprecise manufacturing and unaccounted loads or stiffness. An interconnect system must accurately transmit a plurality of signals, yet be efficient and precise to manufacture and to mount, have minimum mass, stiffness, plastic deformation, and thickness.
Description of Related Art
Traditional HSA interconnect systems use either copper wires or flexible circuits. Wire interconnects utilize manually routed small insulated copper wires (44 AWG and greater) threaded into non-conductive PTFE tubing (0.25 mm-0.38 mm). Enclosing the insulated copper wires within the tubing provides protection from potentially damaging vibrational contact with the suspension member. The PTFE tube typically extends from the distal end of the suspension assembly along the center or the side rails, top or bottom surface of the suspension, to the proximal end of the suspension assembly and beyond. To avoid greatly reducing the dynamic performance and flight characteristics of the slider, only the fine wires are manually curved past the suspension out to the head's electrical terminals to form a service loop.
Wires encased in tubular insulation sheaths (hereinafter wires) possess the advantage that they can be manufactured separately from the suspension assembly and can be added in the final steps of manufacture. They also can adapt to a wide variety of head suspension assembly configurations and topographies and are fairly resistant to breakage and short circuits.
Wires cause problems due to their varying inductances, high profile, and demand for labor and machine intensive processes. Manual conductor routing along the suspension assembly and service loop, can induce unfavorable and unpredictable static slider bias or bias variability. If the wires are not securely fastened along their length to the surface of the suspension assembly they may flex and move. This movement causes variable electrical characteristics. If the service loop is too loose, the wires may snag and reduce the Z-axis clearance. If it is too tight, the wires introduce mechanical stresses and loads in the system. Lack of orientation, placement, and spacing definition of free form twisted conductors at bond sites increases bias variability and bond positioning labor. Usage of suspension appendages termed "wire-crimps", to anchor PTFE tubular wire sheaths, have the potential of inducing stresses to the suspension that affect static roll and pitch. They also may damage the twisted conductors. Profile limitations imposed by PTFE tubing and wire crimp tabs can limit disk stack height.
Individual positioning and bonding of conductors to the slider and amplifier/signal processing electronic cable requires significant labor. As the assembly of head suspension assemblies becomes more automated, the manual routing/bonding of the tube and wire interconnect poses an obvious constraint.
Wires attached along the length of the suspension assembly also may affect the suspension assembly's stiffness and flexibility. The oxygen free copper usually used for the twisted wires has a very low yield strength and has a tendency to plastically deform during assembly and handling, subjecting the suspension assembly to unaccounted stresses. In addition, the constant diameter of the wire does not allow for changes in resistance along certain regions of the suspension assembly. This imposes design limitations due to the trade-off between the fixed inherent electrical resistance (per unit length) of the wire and the need for reduced wire spring stiffness (a function to the fourth power of the wire diameter) along the flexible regions.
Recent developments involve removing the PTFE tubing and using a twisted wire pair bonded in spots to the suspension assembly with an adhesive. This eliminates the added cost and height associated with the PTFE tubing. However, in order to ensure flatness, straightness, and to improve the repeatability and placement accuracy of tubeless wire interconnects, complex automated machinery, such as tensioning devices, are required.
Another common interconnect system uses electrical flex circuits. Electrical flex circuits are flat or round conductive wires laminated within plastic film layers. One of the most common flex circuit embodiments consists of a flat layer of soft copper approximately 18 micrometers to 35 micrometers thick placed on top of an insulating substrate, usually a polyimide material 25 micrometers thick. Flex circuit interconnects are often adhesively bonded to the suspension assembly.
Flex circuits may follow the surface topology of the suspension assembly and may be attached to the suspension assembly all along their length or in selected regions. The resistance along the path of the flex circuit can also be increased or reduced by increasing or reducing the width of the conductors as they transit through certain areas. Flex circuits can be placed on the suspension assembly by automation, have low profiles, controlled impedances, spaced leads, and favorable dynamic response.
But flex circuits also create several problems. The copper traditionally used in flex circuits is prone to plastic deformation, and may introduce unexpected loads on the head suspension assembly. The film substrates used within flexible circuitry lend a high stiffness value to the interconnect.
The substrate in a flex circuit functions both as an electrical isolator and as a base assembly aid/support. While a thin layer would suffice for electrical isolation, to function as base support the dielectric film has to be substantial enough to have some rigidity to allow handling and processing. The thick substrate necessary to support the copper conductors affects the rigidity and flexibility characteristics of the load beam and the gimbal. Traditional load beams are .apprxeq.5 micrometers thick. The addition of a flex circuit at least 43 micrometers thick placed away from the neutral axis of bend of the load beam, more than quadruples the rigidity along the spring regions. This effect is even more drastic along the gimbal. Since conductors usually run on only one side of the load beam, this increase in stiffness is also not symmetrical. Other traditional materials thin enough to reduce interconnect stiffness are difficult to work with and costly.
Finally, although profile, electrical performance, and automation compatible advantages of flex circuitry prove better than those of wires, high volume manufacture of flex circuits lacks precision and is costly. Flex circuits are very difficult to manufacture to specifications similar to suspensions. Polyimide as used as a flex circuit substrate can be mass produced to feature tolerances of .+-.50 micrometers. Exact shaping and matching to the surface topology of an irregular suspension assembly is difficult to achieve. Fine feature patterning techniques of the dielectric substrates (e.g., laser patterning, caustic etching, dry etching) are costly and difficult to implement for large scale production.
A third type of suspension assembly interconnect utilizes plastic compounds either complementing the function of (such as a thin film overlay) or acting as an integral element of the suspension assembly structure. The conductive elements of the interconnect can either be heat staked or molded into the plastic structure. This provides a low profile, favorable dynamic response, and protective attachment to the suspension assembly.
A major problem with the use of plastic materials is that plastics lack the optimal characteristics for use in load beam and gimbal construction. As the flying height and head size continually decrease in the progression towards greater disk storage density, the accuracy and control needed to align the transducer to the correct data track upon the disk surface follows suit. The use of thermally expansive plastics as structural elements of the load beam or gimbal region poses dimensional stability limitations. Heavily interweaved designs of the interconnect and suspension assembly lead to restrictive design constraints on the suspension assembly and/or rigid disk drive designers. Also, since the electrical conductors in flex circuits must be formed on the suspension assembly early in the manufacturing process, the conductors are susceptible to damage from welding, etching, and other manufacturing steps. As a result, the final yield of usable head suspension assemblies is diminished.